I don`t figure out what is VertexStart and VertexCount. For example I have:

Face number

0

1

2

3 Vert in IndxBuff

40,20,24

20,92,42

43,34,25

92,63,24

What is VertexStart? This is vert 40 from face 0 or min vert indx 20 which is in face 0 and face 1? That is VertexStart is min index of vert in index buffer or first index vert in indx buffer?

Furthemore what is VertexCount? This is value 12, that is we have 4 face by 3 vert each = 12. Or this value = 10 because vert 20 repeat twice in face 0 and face 1 and vert 24 repeat twice in face 0 and face3 ?

To allow you to do things like this, all rendering libraries allow you to specify a start index and length with the rendering calls.

They normally also allow for combining meshes into a single vertex buffer for performance, so if you had a single triangle it's indices would obviously be 0,1,2 but if you had already added 100 verts to the buffer you need them to be 100,101,102.

That's all it is, it allows you to lump stuff together so you are not constantly sending vertex buffers across the bus into the graphics card